Entropy 2012, 14, 1501-1521; doi:10.3390/e14081501

entropy ISSN 1099-4300

www.mdpi.com/journal/entropy

Article

Potential and Evolution of Compressed Air Energy Storage: Energy and Exergy Analyses

Young-Min Kim 1,*, Jang-Hee Lee 1, Seok-Joon Kim 1 and Daniel Favrat 2

1 Department of Environmental and Energy Systems, Korea Institute of Machinery and Materials,

171 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea; E-Mails: jhlee@kimm.re.kr (J.-H.L.);

sjkim@kimm.re.kr (S.-J.K.) 2 Industrial Energy System Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL),

Station 9, 1015 Lausanne, Switzerland; E-Mail: daniel.favrat@epfl.ch

* Author to whom correspondence should be addressed; E-Mail: ymkim@kimm.re.kr;

Tel.: +82-42-868-7377; Fax: +82-42-868-7305.

Received: 25 June 2012; in revised form: 8 August 2012 / Accepted: 8 August 2012 /

Published: 10 August 2012

Abstract: Energy storage systems are increasingly gaining importance with regard to their

role in achieving load levelling, especially for matching intermittent sources of renewable

energy with customer demand, as well as for storing excess nuclear or thermal power

during the daily cycle. Compressed air energy storage (CAES), with its high reliability,

economic feasibility, and low environmental impact, is a promising method for large-scale

energy storage. Although there are only two large-scale CAES plants in existence, recently,

a number of CAES projects have been initiated around the world, and some innovative

concepts of CAES have been proposed. Existing CAES plants have some disadvantages

such as energy loss due to dissipation of heat of compression, use of fossil fuels, and

dependence on geological formations. This paper reviews the main drawbacks of the

existing CAES systems and presents some innovative concepts of CAES, such as adiabatic

CAES, isothermal CAES, micro-CAES combined with air-cycle heating and cooling, and

constant-pressure CAES combined with pumped hydro storage that can address such

problems and widen the scope of CAES applications, by energy and exergy analyses.

These analyses greatly help us to understand the characteristics of each CAES system and

compare different CAES systems.

OPEN ACCESS

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Keywords: compressed air energy storage (CAES); exergy; adiabatic CAES; isothermal

CAES; air cycle heating and cooling

Nomenclature c: specific heat (kJ/K) COP: coefficient of performance

c: compression cold: cold water

E: exergy (kJ) e: expansion E+: exergy to the system (kW) ee: energy efficiency E−: exergy from the system (kW) exh: exhaust gas h: specific enthalpy (kJ/kg) g: generator M: mass (kg) h: hydraulic part n: polytropic exponent hot: hot water P: pressure (kPa) Q: heat (kJ) q: specific heat (kJ/kg) r: specific gas constant (kJ/kgK)

HP: heat pump ideal: ideal process in: input to the system m: motor

s: specific entropy (kJ/kgK) net: net (output minus input) T: temperature (K) out: output from the system V: volume (m3) p: conventional power plant v: specific volume (m3/kg) R: refrigeration W: work (kJ) S: storage w: specific work (kJ/kg) s: isentropic Greek Symbols se: electrical storage efficiency

: efficiency (dimensionless) t: isothermal : density (kg/m3) II: second law efficiency

Subscripts 0: dead state (ambient) a: air 1: start state of charging process ac: after compression 2: end state of charging process ae: after expansion Superscripts a(M): mechanical exergy of air +: input a(T): thermal exergy of air −: output

1. Introduction

Interest in energy storage is currently on the rise, especially for storage that matches intermittent

renewable energy with customer demand, as well as for storage of excess nuclear or thermal power

during a daily cycle [1]. In the case of pumped hydro storage, its dependence on specific geological

formations and environmental concerns associated with it make new development difficult. As an

alternative to pumped hydro storage, compressed air energy storage (CAES), with its high reliability,

economic feasibility, and low environmental impact, is a promising method of energy storage [2,3].

The idea of storage plants based on compressed air is not new. In 1978, the first CAES plant of

290-MW capacity was built at Huntorf in Germany. In 1991, another 110-MW plant was built in

McIntosh, AL, USA. Both plants are still in operation [4,5].

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However, the existing CAES plants have some disadvantages such as their energy loss due to

dissipation of heat of compression, use of fossil fuels, and dependence on geological formations.

Therefore, some innovative concepts of CAES have been recently proposed [6]. In the case of

conventional diabatic CAES plants, because the heat of compression needs to be dissipated to prevent

deterioration of the cavern and pressure drop of the stored air in the cavern by heat loss, natural gas is

used to heat the compressed air during the discharging process in order to avoid extremely low

temperature of turbine. But adiabatic CAES stores the heat of compression and reuses it to heat the

compressed air during the discharging process. The efficiencies of several adiabatic CAES

configurations are analyzed with the help of an energy balance [7,8]. Isothermal CAES can also

minimize the energy loss that occurs during the charging and discharging processes by minimizing the

temperature difference of the air with the environment [4,9]. Micro-CAES with an air cycle heating

and cooling system can use the heat of compression and the cooling effect of expanding air with good

efficiency as a multipurpose system [10]. Constant-pressure CAES combined with pumped hydro

storage can eliminate throttling loss in the existing CAES plants with varying air storage pressure and

reduce the volume of air storage [11,12].

This paper reviews the main drawbacks of the existing CAES systems and the advantages of

innovative CAES concepts such as adiabatic CAES, isothermal CAES, micro-CAES combined with

air-cycle heating and cooling, and constant-pressure CAES combined with pumped hydro storage by

energy and exergy analyses. These analyses greatly help us to understand the characteristics of each

CAES system and to compare different CAES systems.

2. Understanding CAES System Using Exergy Concept

CAES is not a simple energy storage system, like other batteries. It can be viewed as a hybrid of an

energy storage and a gas turbine power plant. Unlike conventional gas turbines, which consume about

two-thirds of their input fuel to compress the air at the time of power generation, CAES precompresses

the air, using low-cost electricity from the power grid at off-peak times, and utilizes it with some gas

fuel to generate electricity when required [13]. The compressed air is stored in appropriate

underground caverns or aboveground air vessels. The schematic of a modern CAES facility is

presented in Figure 1 [9].

Figure 1. Conventional CAES system: Schematic of the McIntosh plant in AL, USA.

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Since the CAES system can be considered to be a combination of an energy storage system and

power plant, and all processes are accompanied by a significant amount of heat transfer, understanding

the electrical energy flow in the CAES system is very difficult using the first law of thermodynamics,

which deals with heat and work equally; therefore, we can use exergy flow, which is based on the

second law of thermodynamics, to better understand the characteristics of the CAES system. Exergy

can be defined as the maximum useful work possible that is accomplished during a process that brings

the system into ion equilibrium with the environment [14]. Assuming that potential and kinematic

energy effects are negligible and no chemical reaction occurs, the exergy of an air stream can be

expressed as [15]:

0 0 0a aE M h h T s s (1)

where h and s are specific enthalpy and entropy, respectively, and the subscript 0 indicates that the

properties are taken at the environmental temperature and pressure ( 0T = 20 °C, 0P = 1 bar).

In the case of perfect gas flow:

0 0ph h c T T (2)

00 0

ln lnp

T Ps s c r

T P

(3)

where pc is isobaric specific heat at average temperature and r is the specific gas constant.

The exergy in Equation (1) can be split into two parts—the mechanical part and the thermal part—

as follows [14]:

( ) ( )a a M a TE E E (4)

00)( ln

P

PrTME aMa

(5)

( ) 0 00

lna T a p

TE M c T T T

T

(6)

Figure 2 shows the energy and exergy flow of the simplified CAES system. The processes of the

CAES system are compression (1–2), cooling (2–3), charging or discharging (3–4), heating (4–5),

expansion (5–6), and recuperation (6–7).

Although the compression work varies according to various compression processes, e.g.,

isothermal, adiabatic compression, and two-, three-, and four-stage compression with intercooling, the compressed air is cooled and stored at the same environmental temperature ( ) and storage pressure

( ). This means that only the mechanical exergy of the compressed air is stored in the air storage,

and the thermal exergy of the compressed air is dissipated by the cooler or can be stored in the thermal

energy storage and reused during the discharging process.

0T

SP

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Figure 2. Energy and exergy flow of simplified CAES system.

eE

cQ

cE

eQ

aMaM

aM

00 , PTSP

)(MaE

)(TaE

)(TaE

aV

exhQ

)(TaE

The isentropic efficiencies of the compressor and expander, sc, and se, , respectively, are

expressed as:

, out, in,

out in

c s sc s

c

W h h

W h h

(7)

in out,

, in out,

ee s

e s s

W h h

W h h

(8)

where the subscript s denotes an isentropic compression/expansion process. The efficiencies of motor and generator, m and g , are expressed as:

c

cm E

W (9)

e

eg

W

E

(10)

In order to study the energy and exergy analyses of different types of CAES systems, we adopted

the following assumptions and elements of inlet data:

(a) Each compression and expansion is assumed to occur at a steady state with a steady flow and

constant-pressure air storage, have negligible potential and kinematic energy effects, and exhibit

no chemical reaction;

(b) For simplicity, the pressure drop in all heat exchangers is neglected;

(c) Air pressure in the storage cavern (or vessel) is assumed to remain constant at 5 MPa by

adoption of constant-pressure air storage [10,11]. The designed pressure ratios of the

compressor and expander are 50, with isentropic (adiabatic) efficiencies of 85%. The air-storage

pressure is optimized by energy density and efficiency of the system and the general value of

air-releasing pressure for CAES gas turbine is around 5 MPa [10,11];

(d) The efficiencies of the motor and generator are assumed to be 95%.

For multistage compression, each stage was assumed to consist of an adiabatic compression

process, and each stage had the same pressure ratio; the compressed air was cooled to the ambient

Entropy 2012, 14

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temperature by intercooling. Moreover, the total compression work and heat rejection are the sums of

individual values of these quantities at all stages. For the discharging process, the recuperator can be

used for recovering the heat of the expanded hot gas.

3. Characteristics of Different Types of CAES Systems

3.1. Conventional Diabatic CAES System (with Fuel)

In the case of conventional CAES plants such as the one shown in Figure 1, in order to increase the

overall efficiency of the system, it is customary to perform multistage compression with intercooling

and multistage expansion with reheating, and for the discharging process, the compressed air is

generally heated using gas fuel. Figure 3 shows the energy and exergy flow of the diabatic CAES

system for the production of 1-kWh output.

Figure 3. Energy and exergy flow of the conventional CAES system.

)kWh 0.63(cE

aMaM

aM

00 , PT)bar 50(SP

)kWh 44.0()(MaE

)kWh 09.0()(

TaE

)m081.0( 3aV

)kWh 00.1(eE

)kWh 62.0(cQ )kWh 22.1(eQ

)kWh 77.0()(

TaE

)kWh 16.0(exhQ

)kWh 02.0()(TaE

The CAES system in Figure 3 is based on the McIntosh plant, which has four-stage compression

with intercooling and an exhaust gas temperature of 130 °C. The heat input is 1.22 kWh to produce

1 kWh electric output, and the inlet temperatures of high pressure (HP) turbine with the inlet pressure

of 50 bar and low pressure (LP) turbine with the inlet pressure of 15 bar are 537 °C and 871 °C,

respectively. The electric power is 2.26 times higher than the exergy of the compressed air by the

thermal exergy added by using fuel. The thermal exergy of 0.77 kWh in Figure 3 is not chemical

exergy but a total thermal exergy added in form of heat during the multistage expansion with reheating.

In general, the energy efficiency of the CAES system can be calculated by adding the heat input by natural gas, Q , to the electric input as follows [13,16,17]:

(11)

But, this value may not be considered a useful measure of electrical storage efficiency, because the

heat input is not equivalent to the electric input. Rather, it may be seen as a mixed efficiency of the

electrical storage efficiency and thermal efficiency of a power plant.

out

inee

E

E Q

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Therefore, the net electrical storage efficiency of a CAES system can be calculated by adding the

amount of electric energy generated by natural gas (or other fuels) to the electric input of the CAES

system as follows [13,16,17]:

(12)

where pQ is the electric energy that would be produced if the amount of fuel consumed by the CAES

system were burned in another power station (Q is the lower caloric value of the fuel and p is the

efficiency of the conventional power plant).

If the standard thermal efficiency of the power plant 4.0p [18] is assumed, the net electrical

storage efficiency of the CAES system shown in Figure 3 is 89%. However, if the thermal efficiency

of a conventional power plant, 5.0p , is used as the standard, the net electrical storage efficiency of

the CAES system is 81%. Therefore, the net electrical efficiency of the CAES system, defined in

Equation (12), depends significantly on the value of p used. Further, when the heat is obtained from

waste heat or solar power, the net electrical storage efficiency of a CAES system becomes more

ambiguous. Figure 4 shows a new configuration of diabatic CAES configuration, one that uses a

conventional gas turbine and the exhaust heat from the gas turbine to heat the compressed air before it

reaches the expander, as a CAES bottoming cycle [19].

Figure 4. Configuration of CAES system as a bottoming cycle.

Therefore, in this paper, a new expression for the electrical storage efficiency of the CAES system,

based on the exergy analysis, has been proposed. It is given as:

(13)

where )(TaE is the thermal exergy of compressed air added by heat sources.

The new electrical storage efficiency of the CAES system in Figure 3 is 71%. Unlike the

conventional gas turbine system, in the case of the CAES system, there is a very little exergy loss of

exhaust gas. This advantage can compensate for the exergy loss of heat of compression. Although the

heat of compression is dissipated in existing CAES plants, it is possible for this dissipated heat to be

used for residential heating through cogeneration, which can contribute to improved energy efficiency.

out

inse

p

E

E Q

out

in ( )se

a T

E

E E

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Moreover, although existing CAES plants use fossil fuel, waste heat and several other types of

renewable energies such as solar, biomass, and biogas can be used as heat sources in the CAES

system. SolarCAT Inc. has developed an energy storage technology using compressed air energy

storage and concentrating solar power (CSP) design concepts. The company’s design replaces the

CAES facility’s expander train with multiple expanders integrated with parabolic dish solar collectors,

which reflect and concentrate incident sunlight onto their focal receivers to heat the compressed air fed

to the collectors from storage [4].

3.2. Adiabatic CAES System

One of the drawbacks of existing CAES systems is the use of fuel. Adiabatic CAES uses no fuel to

heat the compressed air for the expansion process, representing an emission-free, pure storage

technology with high storage efficiency [4]. The basic idea of the adiabatic CAES concept is the use of

heat storage as the central element of the plant, as shown in Figure 5 [4,20,21]. This implies that the

heat needed to heat the compressed air for the expansion process is recovered from the compression

and stored in a thermal energy storage (TES) unit to eliminate the need for a combustor [13].

Figure 5. Diagram of an adiabatic CAES system in single-stage configuration.

00 , PT

)bar 50(SP

Figure 6 shows the energy and exergy flow of the adiabatic CAES system in single-stage

configuration for the production of 1 kWh output. If the air is compressed up to 50 bar adiabatically, the outlet air temperature of compressor is about

685 °C. The thermal exergy of the compressed air is roughly equivalent to the mechanical

exergy of the compressed air . During the charging process, the heat is extracted from the

compressed air and stored. When the grid demands electric power, the compressed air is heated using

the thermal energy storage up to 622 °C and expanded through an air turbine. It was assumed that the

efficiency of TES (the ratio of heat input to the compressed air to the heat output from the compressed

air) is 90%. Then, the electrical storage efficiency of the adiabatic CAES system without any external

thermal input is 68%. The adiabatic CAES can be considered a combination of CAES and TES,

because a substantial portion of the exergy is stored in the form of thermal energy.

( )a TE

( )a ME

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Figure 6. Energy and exergy flow of the adiabatic CAES system in single-stage configuration.

kWh) 1.48(cE

aMaM

aM

00 , PT)bar 50(SP

)kWh 65.0()(MaE

)kWh 69.0()(

TaE

)m 119.0( 3aV

)kWh 00.1(eE

)kWh 43.1(cQ )kWh 28.1(eQ

)kWh 59.0()(

TaE

)kWh 21.0(exhQ

)kWh 03.0()(TaE

Adiabatic CAES technology was evaluated previously in the European research project. The

resulting conceptual designs of the four main plant components (compressor, heat storage, cavern, and

air turbine) helped to identify some key technical risks as well as a substantial need for further

development efforts, particularly for the adiabatic compression and the heat storage device. The outlet

temperature of the compressor is maintained above 600 °C, and the corresponding compression heat is

stored in the TES consisting of an arrangement of solid materials, typically ceramic bricks or natural

stones [4,21].

Another configuration of adiabatic CAES is shown in Figure 7, using cold and hot oil as the TES [7,8].

During charging process, the oil is flowing from a cold tank to a hot tank to intercool the compressed

air. Inversely, during discharging process, the oil is flowing from the hot tank to the cold tank to heat

the compressed air for power production. The design is based on using standard, industry-proven

equipment components.

Figure 7. Diagram of an adiabatic CAES system in two-stage configuration.

00 , PT

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Figure 8 shows the energy and exergy flow of the adiabatic CAES system in two-stage

configuration for the production of 1 kWh output.

Figure 8. Energy and exergy flow of the adiabatic CAES system in two-stage configuration.

kWh) 1.50(cE

aMaM

aM

00 , PT)bar 50(SP

)kWh 90.0()(MaE )m 164.0( 3aV

)kWh 00.1(eE

)kWh 45.1(cQ )kWh 20.1(eQ

)kWh 11.0(exhQ

)kWh 01.0()(TaE

)kWh 41.0()(

TaE )kWh 34.0()(

TaE

If the air is compressed up to 50 bar in two stages, the outlet air temperature of compressor is about

275 °C. The thermal exergy of the compressed air, ( )a TE , is less than half of the mechanical exergy of

the compressed air, ( )a ME . During the charge period, the heat is extracted from the compressed air and

stored by the flowing oil. When the grid demands electric power, the compressed air is heated using

thermal energy storage up to 250 °C and expanded through an air turbine. The efficiency of TES is

assumed to be 90%, as in the previous adiabatic CAES system in single-stage configuration. Then, the

electrical storage efficiency of the adiabatic CAES system without any external thermal input is 67%.

This adiabatic CAES configuration needs greater air storage than does the prior one, because its TES

exergy storage is much smaller than the prior one, due to the lower temperature of TES.

3.3. Isothermal CAES System

Isothermal CAES is an alternative CAES system which eliminates the need for fuel and high

temperature thermal energy storage. Isothermal CAES can minimize the compression work and

maximize the expansion work done through isothermal compression/expansion by means of effective

heat transfer with the vessel’s surroundings, which involves slow gas pressure change by liquid

piston [4,9]. The CAES systems of SustainX Inc. and Enairys Powertech Ltd. use hydraulic pumps to

isothermally compress air at rates that allow the high-pressure air to exchange heat with its

surroundings, as shown in Figure 9. During isothermal expansion, the gas is able to do more work by

absorbing heat from its surroundings. The expanding air in the isothermal CAES system drives a

hydraulic motor, which in turn drives an electric generator. The use of hydraulic pumps and motors

enables precise compression and expansion of air, heat transfer with the surroundings, and a high level

of thermal and overall system efficiency [4,9].

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Figure 9. Function diagram of an isothermal CAES system (Source: SustainX Inc.,

Seabrook, NH, USA).

Figure 10 shows the energy and exergy flow of the isothermal CAES system for the production of

1 kWh output. The air is compressed up to 50 bar isothermally and stored; then, the compressed air is

expanded isothermally. The ideal isothermal compression and expansion work, ,c tW and ,e tW ,

respectively, can be obtained as follows:

(14)

(15)

where the working fluid is assumed to be real gas then h is very small but not equal to zero.

Figure 10. Energy and exergy flow of the isothermal CAES system.

)kWh 1.37(cE

aMaM

aM

00 , PT)bar 50(SP

)kWh 17.1()(MaE )m 213.0( 3aV

)kWh 00.1(eE

)kWh 34.1(cQ )kWh 09.1(eQ

)kWh 0(exhQ

)kWh 0()(TaE

)kWh 00.0()(

TaE )kWh 00.0()(

TaE

The isothermal efficiencies of compression and expansion, and , respectively, are

expressed as:

(16)

(17)

where cW and

eW are the actual compression work and expansion work considering a loss of

mechanical work by internal dissipation and a small temperature deviations from isothermal process.

)()( ,, hsTMhqMW catcatc

)()( ,, hsTMhqMW eateate

tc, te,

,,

c tc t

c

W

W

,,

.ee t

e t

W

W

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It was assumed that the isothermal efficiencies of compression and expansion were 90%,

considering the high efficiency of the hydraulic pump/motor and the effective heat transfer with its

surroundings; the isentropic effciencies of hydraulic water pump/motor are 92%~94% [9] and it was

repoted that the temperature deviation during the expansion (or compression) process from 1 bar to

200 bar is less than 12 °C by water injection, which means that the work by integrating the pressure-

volume curve is 96% of isothermal work (source: SustainX Inc.). Although there is much heat transfer between the system and its surroundings for the compression

and expansion processes, the thermal exergy of the air )(TaE is nearly equal to zero, because the

temperature change of the air is very small. Although the isothermal CAES system requires about

twice the volume of air storage in comparison with the conventional (diabatic) and adiabatic CAES

systems, because no thermal exergy is added to the compressed air for the discharging process, the use

of high pressure (from 1 bar to 200 bar) reduces the storage volume required. The electrical storage

efficiency of the isothermal CAES system without external thermal input is 73%.

3.4. Trigeneration Micro-CAES System

Large-scale CAES is very dependent on appropriate geological formations for air storage.

Micro-CAES with subsurface or aboveground pressure vessels is a more adaptable solution, especially

for distributed generation [22–24]. Also compressed air is a commonly used utility across most

manufacturing and processing industries as its production and handling are safe and easy [25].

Micro-CAES systems can be used as a multipurpose system, as a combination of energy storage and

air cycle heating and cooling. In general, air cycle refrigeration consists of one or more compressors

(fan), turbines (expanders), and heat exchangers; the setup is very similar to the CAES system except

for the air storage vessels. Air cycle heating and cooling has many advantages, including high

reliability, ease of maintenance, flexibility of temperature, and heating and cooling capacities. In

addition, it employs a natural refrigerant, which is environmentally benign [26–29]. The air cycle

approach is one of the most promising long-term alternatives for refrigeration machines, air

conditioners, and heat pumps [26]. Air cycle is widely used in industrial freezing with a very low

temperature (below −50 °C) and transport refrigeration. In the case of conventional refrigeration and

air conditioning, the air cycle approach suffers from low energy efficiency and so is not competitive

with vapor compression cycles [27]. But, air cycles have been studied for HVAC in office buildings

within the framework of European research projects, since they have advantages in applications that

require both heating and cooling [26,29] and because of the potential of highly efficient turbo

machinery that is used [27,28].

The micro-CAES system combined with air cycle heating and cooling could be a very good

solution because it would then be possible to improve the energy efficiency and economics of the

system as a multipurpose system. Transportable CAES was proposed as a solution to provide power

and energy for desalination of brackish water or seawater for an island by utilizing the superchilled air

as a by-product for the freeze-crystallizing process [30]. A new CAES refrigeration system was

proposed for electrical power load shifting—one that employs a combination of an air cycle

refrigeration cycle and a vapor compression cycle, in which the cooling effect of expanding air is used

and the expansion power is used to drive a compressor of the vapor compression cycle [31]. This paper

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focuses on a micro-CAES system for energy storage and air cycle heating and cooling for HVAC of a

building. Because the pressure ratio of CAES is much higher than that of conventional air cycle

refrigeration to reduce the volume of air storage, multiple-stage compression and expansion is needed

to achieve high efficiency for the system; this is effected by matching the temperature of compressed

and expanded air to the required temperature of heating and cooling load. Otherwise, quasi-isothermal

compression and expansion by liquid injection can be used for a high efficiency micro-CAES and air

cycle heating and cooling system [10]. To achieve quasi-isothermal compression and expansion, a

large amount of atomized water is injected during both processes, as shown Figure 11.

Figure 11. Trigeneration micro-CAES system: (1) mixer, (2) compressor, (3) separator, (4)

hydraulic motor, (5), (6), and (7) heat exchanger, (8) high pressure vessel, (9) mixer, (10)

expander, (11) separator, (13) pump, and (12), (14), and (15) heat exchanger [10].

8

2 10

Motor Generator

31

5 9 12

4 1311

Qc

Qac

Qe

Qae

7 614

AirInlet

AirOutlet

15

The concept is the same as that of a liquid-flooded compressor and expander in an Ericsson cycle

cooler proposed by Hugenroth et al. [32]. Water (or liquid) is discharged together with compressed air;

then it is separated, cooled, and recirculated. Compressed air is cooled to the ambient temperature by

water circulation through a cooler, and it is then stored in vessels. Hot water separated after the

compression and hot water from the cooler can be used to satisfy the heating load. To achieve

quasi-isothermal expansion, as in the compression process, liquid can be injected into the expander,

separated, used to supply the cooling load, and pressurized by a pump for injection. In the course of the

analyses, additional compression work owing to liquid during the compression process or pumping

work for liquid injection during the expansion process was neglected, because either kind of work

supplies only a small percent (2%–3%) of the work of gas, and the additional work input owing to

liquid can be recovered by hydraulic motor during the compression process or by expander during the

expansion process.

The quasi-isothermal compression or expansion process can be modeled as a polytropic process

(Pn = const). The temperature change of gas during the polytropic process can be obtained as follows:

(18)

n

n

in

out

in

out

P

P

T

T1

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In the case of quasi-isothermal compression or expansion, it is possible to match the temperature

after compression to the heating temperature required or the temperature after expansion to the cooling

temperature required by controlling the mass flow rate of injected liquid, i.e., by changing the

polytropic index n.

The ideal compression and expansion work can be obtained as follows:

(19)

(20)

Actual compression and expansion work can be obtained using an isentropic efficiency defined,

respectively, as:

(21)

(22)

Heat transfer from the gas to the liquid during the compression process and from the liquid to the

gas during the expansion process can be obtained as follows:

(23)

(24)

Figure 12 shows the energy and exergy flow of the trigeneration micro-CAES system for the

production of 1 kWh output. The case of one stage of quasi-isothermal compression with (n = 1.05)

and expansion (n = 1.025) with liquid injection was considered. It was assumed that the isentropic

efficiencies of the compressor and expander are 85%, the efficiencies of the motor and generator are

95%, and we can use 90% of the heat of compression and cooling effect of expanding air. The

electrical storage efficiency of the micro-CAES system without any external thermal input is 57%. In

addition, hot and cold water accompanied by a significant amount of heat transfer are available, in

which the COP of the heat pump (COPHP = Qhot/Enet) is equivalent to 2.0 and the COP of refrigeration

(COPR = Qcold/Enet) is equivalent to 1.3. Considering the thermal exergy of the hot and cold water, the second law efficiency of the trigeneration micro-CAES system ( II ) is 68%.

Figure 12. Energy and exergy flow of trigeneration micro-CAES system.

inoutidealc TTn

rnw

1,

outinideale TTn

rnw

1,

c

idealcc w

w ,

ideale

ee w

w

,

)( inoutcac hhwMQ

)( outineae hhwMQ

Entropy 2012, 14

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To obtain increased electric power without use of the cooling effect, a heater (or combustor) can be

used to heat the compressed air. Figure 13 shows the energy and exergy flow of the micro-CAES

system with fuel, as in the conventional CAES system.

Figure 13. Energy and exergy flow of micro-CAES system with the fuel.

3.5. Constant-Pressure CAES System Combined with Pumped Hydro Storage

Conventional CAES systems are most commonly operated under constant volume conditions with a

fixed, rigid reservoir operating over an appropriate pressure range [15,20]. These varying pressure

ratios can degrade the efficiencies of compression and expansion due to deviation from design points.

Existing CAES plants were designed to throttle the cavern air to a designed pressure despite the

throttling loss. The exergy loss of compressed air by throttling is about 5%–8% in existing CAES

systems [11]. Although it is possible to increase the storage volume to reduce the operating pressure

range, doing so results in low energy density and high construction costs. Therefore, in order to resolve

such problems, a new constant-pressure CAES system combined with pumped hydro storage was

proposed [11,12]. The system combines constant-pressure air storage and hydraulic energy storage, as

shown in Figure 14. During the charging process, the water in an air storage vessel (left) is transferred

to a hydraulic accumulator (right) by a pump to maintain a constant pressure of air storage, consuming

power. During the discharging process, the water in the hydraulic accumulator is returned to the air

storage vessel through a water turbine to maintain a constant pressure of air storage, producing power. The compression/expansion process of the enclosed air in the right tank is assumed to be a

isothermal process (PV = const) that results from sufficient heat transfer with the environment

considering that the pressure ratio of the sealed air, P2/P1, is low and the compression/expansion

process is a very slow process over the charging/discharging process. The exergy of compressed air

part ( ) and hydraulic part ( ) can be expressed, respectively, as [11]:

(25)

(26)

aE hE

021 ln)(

P

PVVPE S

Sa

)(ln 212

111 VVP

V

VVPE Sh

Entropy 2012, 14

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where V1 and V2 are the volumes of air in the hydraulic accumulator at the start and end of the charging

process, respectively; P1 is the pressure of air in the hydraulic accumulator at the start of charging

process; and P0 and PS are ambient pressure and constant pressure of air storage, respectively.

Figure 14. Constant-pressure CAES system combined with pumped hydro storage (PHS) [11].

This concept can be applied to both small-scale CAES using manmade air vessels and large-scale

CAES using salt caverns or lined rock caverns. In the case of small-scale CAES systems, a commercial

hydraulic motor/pump can be used because the machines exhibit exceptional energy conversion

performances and are easily reversible (can operate as motor and as pump) [9]. In the case of micro-

CAES, it is very important to increase the energy density and reduce the volume of the storage tank at

a feasible cost, because of the high cost and space of the storage tank. Recently, a CAES system of the

Electric Power Research Institute Inc. with aboveground air vessels throttled the incoming air to 55 bar

(operating between 55 bar and 103 bar), despite the big throttle loss. The constant-pressure CAES

system combined with pumped hydro storage can be applied in the micro-CAES system with a

manmade air vessel to increase the energy density and efficiency of the system.

If it is assumed that the compression volume ratio (V1/V2) is 4 and the constant pressure of air

storage (PS) and initial pressure of air in the hydraulic accumulator (P1) are about 50 bar, then

approximately 80% exergy is stored in the compressed air part and 20% exergy is stored in the

hydraulic part. Figures 15 and 16 show the energy and exergy flow of the previous micro-CAES

system applied by the constant-pressure CAES system combined with pumped hydro storage. The

exergy efficiencies of the water pump and water turbine were assumed to be 0.9.

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Figure 15. Energy and exergy flow of constant-pressure trigeneration micro-CAES system.

Figure 16. Energy and exergy flow of constant-pressure micro-CAES system with the fuel.

In the case of a large-scale CAES system, it is very important to build storage caverns economically

using natural geological formations such as salt domes or hard rocks. The constant-pressure CAES

system combined with pumped hydro storage can be considered a combination of CAES and a new

type of underground pumped storage. Underground pumped storage has been studied as an alternative

to conventional aboveground pumped storage, requiring available topography and consideration of

environmental impact. The underground pumped storage can be constructed by setting the upper

reservoir aboveground and the lower reservoir and powerhouse underground [33,34]. In the case of

conventional underground pumped storage, the unit cost of the excavation increases with depth, a deep

borehole was drilled into the rock strata below the mine where the powerhouse would be located. Also,

Entropy 2012, 14

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the excavation for the power facilities and installations would add a major cost to the project and

therefore significantly increase the construction cost [34].

The advantages of the constant-pressure CAES system combined with pumped hydro storage are

that it is not dependent on depth; the powerhouse can be installed aboveground and there is no need of

an upper reservoir aboveground, as shown in Figure 17. Hydraulic energy storage in the system has

many advantages over the conventional CAES system, including quick start-up, the ability to provide

“spinning reserve,” and voltage and frequency regulation to stabilize the associated power grid [35,36].

Figure 17. Constant-pressure CAES system combined with PHS (aboveground power house).

SP

Table 1. Energy and exergy analyses of different types of CAES systems [units: cE ,

cQ ,

)(TaE , )(MaE , eQ ,

)(TaE , eE (kWh); cT , eT ( ); aV (m3)].

Type

Charging Storage Discharging

se cE

cQ

( cT )

)(TaE )(MaE aV

eQ

( eT )

)(TaE

eE

Diabatic 0.63 0.62 (115)

0.09 0.44 0.081 1.22 (537,871)

0.77 1.00 0.71

Adiabatic 1-stage

1.48 1.43 (685)

0.69 0.65 0.119 1.28 (622)

0.59 1.00 0.68

Adiabatic 2-stages

1.50 1.45 (275)

0.41 0.90 0.164 1.20 (250)

0.34 1.00 0.67

Isothermal 1.37 1.34 (20)

0.00 1.17 0.213 1.09 (20)

0.00 1.00 0.73

Trigen Micro 1.77 1.55 (80)

0.15 1.29 0.236 0.99 (−6)

0.05 1.00 0.57

COPHP = 2.0, COPR = 1.3, II = 0.68 Micro with fuel

0.60 0.53 (20)

0.05 0.44 0.081 1.22 (537,871)

0.77 1.00 0.73

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4. Conclusions

Exergy analysis was used to provide an understanding of the drawbacks of the current CAES

method and the advantages of innovative CAES concepts such as adiabatic CAES, isothermal CAES,

micro-CAES combined with air-cycle heating and cooling, and constant-pressure CAES combined

with pumped hydro storage. The results of exergy analyses of different types of CAES systems are

summarized in Table 1.

Although existing CAES plants use natural gas as fuel, waste heat and renewable energies such as

solar, biomass, and biogas can be used as heat sources for the CAES system. Adiabatic CAES traps the

heat of compression and reuses it for expansion; hence, it can be considered a combination of CAES

and thermal energy storage (TES), because a substantial portion of the exergy is stored in the form of

thermal energy. Isothermal CAES can minimize the compression work and maximize the expansion

work done through isothermal compression/expansion with effective heat transfer with the system’s

surroundings and without the use of fuel or high temperature thermal storage. The micro-CAES system

combined with air cycle heating and cooling can be a very effective system for distributed power

networks, because it provides energy storage, generates electric power using various heat sources, and

incorporates an air cycle heating and cooling system. Constant-pressure CAES combined with pumped

hydro storage can reduce the throttling loss during expansion in the existing CAES plants; this method

requires a small cavern (or vessel) whose storage volume is only half of that required for existing

CAES plants.

The existing CAES systems have a few drawbacks, but these can be addressed by employing some

innovative CAES concepts. Compressed air energy storage can be combined with power generation

using various heat sources, thermal energy storage, air cycle heating and cooling, and pumped hydro

storage; such combinations have great synergistic effects.

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